The invention relates to an energy storage device having a DC voltage supply circuit and to a method for providing a DC voltage from an energy storage device, in particular a battery direct inverter that supplies an on-board vehicle electrical power supply system with a DC voltage.
The trend is that in the future electronic systems which combine new energy storage technologies with electrical drive technology will be used increasingly both in stationary applications, such as e.g. wind power installations or solar installations, and in vehicles such as hybrid or electric vehicles.
The feed of polyphase current into an electrical machine is usually realized by a converter in the form of a pulse-controlled inverter. For this purpose, a DC voltage provided by a DC voltage intermediate circuit can be converted into a polyphase AC voltage, for example a three-phase AC voltage. The DC voltage intermediate circuit is fed in this case by a string of battery modules interconnected in series. In order to be able to meet the requirements for power and energy provided for a respective application, often a plurality of battery modules are connected in series in a traction battery.
The series circuit comprising a plurality of battery modules is associated with the problem that the entire string fails if a single battery module fails. Such a failure of the energy supply string can result in a failure of the entire system. Furthermore, temporarily or permanently occurring power reductions of an individual battery module can result in power reductions in the entire energy supply string.
The document U.S. Pat. No. 5,642,275 A1 describes a battery system with an integrated inverter function. Systems of this type are known under the name Multilevel Cascaded Inverter or else Battery Direct Inverter (BDI). Such systems comprise DC sources in a plurality of energy storage module strings, which can be connected directly to an electrical machine or an electrical power supply system. In this case, single-phase or polyphase supply voltages can be generated. The energy storage module strings in this case have a plurality of energy storage modules connected in series, wherein each energy storage module has at least one battery cell and an assigned controllable coupling unit, which makes it possible to bridge the respectively assigned at least one battery cell or to switch the respectively assigned at least one battery cell into the respective energy storage module string, depending on control signals. In this case, the coupling unit can be designed in such a way that it additionally makes it possible to switch the respectively assigned at least one battery cell also with inverse polarity into the respective energy storage module string or else to interrupt the respective energy storage module string. By suitable driving of the coupling units, for example with the aid of pulse width modulation, suitable phase signals for controlling the phase output voltage can also be provided, with the result that a separate pulse-controlled inverter can be dispensed with. The pulse-controlled inverter required for controlling the phase output voltage is thus integrated into the BDI as it were.
BDIs usually have a higher efficiency, a higher degree of failsafety and a significantly lower harmonic content of their output voltage in comparison with conventional systems. The failsafety is ensured, inter alia, by virtue of the fact that defective, failed or not fully effective battery cells can be bridged in the energy supply springs by suitable driving of the coupling units assigned to them. The phase output voltage of an energy storage module string can be varied and in particular set in a stepped manner by corresponding driving of the coupling units. In this case, the stepping of the output voltage results from the voltage of an individual energy storage module, wherein the maximum possible phase output voltage is determined by the sum of the voltages of all the energy storage modules of an energy storage module string.
The documents DE 10 2010 027 857 A1 and DE 10 2010 027 861 A1, for example, disclose battery direct inverters having a plurality of battery module strings which can be directly connected to an electrical machine.
A constant DC voltage is not available at the output of BDIs since the energy storage cells are divided among different energy storage modules and the coupling devices thereof have to be driven in a targeted manner for generating a voltage level. As a result of this distribution, a BDI is basically not available as a DC voltage source, for example for feeding an on-board electrical power supply system of an electrical vehicle. Accordingly, the charging of the energy storage cells via a conventional DC voltage source is not readily possible either.
Therefore, there is a need for an energy storage device having a DC voltage supply circuit and a method for operating same with which a DC voltage consumer can be fed with a DC voltage from the energy storage device or a DC voltage charging circuit can be coupled to the energy storage device.